Certain known wind turbines for producing electric energy normally comprise a main frame that rotates about a vertical axis; a blade assembly, which comprises a hub fitted with at least two blades, and is connected to the main frame by at least one bearing to permit rotation of the blade assembly about an axis of rotation; and a rotary electric machine, which is rotated by the blade assembly, and comprises a stator connected to the main frame, and a rotor connected to the blade assembly. Normally, the stator comprises a tubular first active part, and the rotor comprises a tubular second active part facing, and separated by an air gap from, the tubular first active part. The efficiency of a rotary electric machine is known to be affected significantly by the size of the air gap: the smaller the air gap, the greater the efficiency of the electric machine.
On direct-drive wind turbines, the blade assembly is connected rigidly to the rotor, and the bearing serves to support both the rotor and the blade assembly for rotation about the axis of rotation; and a fairly common practice is to employ one bearing designed to absorb radial and axial forces and tipping moments. Other wind turbine configurations employ two coaxial bearings.
The bearings used between the blade assembly and main frame of wind turbines are normally rolling bearings. A rolling bearing comprises an outer ring and an inner ring; and at least one ring of rolling bodies equally spaced about the axis of rotation, between the outer and inner rings. On wind turbines, it is generally good practice to assemble the rolling bearings with a relatively small amount of interference between the inner ring on one side, the outer ring on the other side, and the rolling bodies between the inner ring and outer ring. In other words, the bearing can be assembled to achieve a relatively small amount of both radial and axial interference. Rolling bearings may normally be assembled with interference or clearance; and in the following description, the term ‘preload’ is used to describe both interference and clearance configurations.
The set preload on the bearing at the installation stage may vary widely over time, depending on the temperature range at the wind turbine installation site, and on operating conditions, such as the heat generated by the rotary electric machine, friction in the bearing, and the efficiency of the cooling system. Changes in the preload of the bearing may produce relatively severe stress on the bearing, wear affecting the working life of the bearing, or slack which equally impairs the working life of the bearing and also alters the size of the air gap. Changes in the preload of the bearing therefore have a particularly harmful effect on the working life of the bearing and, in some situations, also on the efficiency of the electric machine.
These drawbacks are further compounded by the current trend towards relatively large-diameter rotary electric machines.
European Patent No. 2,290,250 discloses a method for controlling the temperature of a sleeve supporting two bearing at its opposite ends for a cartridge of drive-train of a geared wind turbine.
European Patent No. 1,992,829 discloses a method for controlling the temperature of bearing in machine tool.
Any one of these documents offers solutions that are not fully satisfactory for a direct drive wind turbine.